docs: def filter(self, *condition):
          """
          If there are multple condition, then treats them *and* relastion.
          """
          if not condition:
              return self
          cond = true()
          for c in condition:
              if c is not None:
                  if isinstance(c, (str, unicode)):
                      c = text(c)
                  cond = and_(c, cond)
          if self.condition is not None:
              self.condition = and_(cond, self.condition)
          else:
              self.condition = cond
          return self